The invention relates to receiver circuitry for handling incoming echo signals of an echo-sounding system and, in particular, deals with the suppression of interfering signals.
Various methods and circuits for suppressing noise and interference signals in echo-sounding systems, and for blanking out false echos, for instance, the so-called secondary echo, are known. For purposes of the following description, in the context of depth metering, the primary echo is that received directly from the bottom of the sea. The secondary echo is generated by reflection of the primary echo from the water surface and then from the seabed.
German Pat. No. 2,535,029 shows an echo-sounding system in which useful signals occurring at successive intervals of approximately the duration of one sounding period are summed up, and the summation is used for display or indication. Noise signals appearing at random are not summed up and can be suppressed by an appropriate threshold circuit. This type of noise suppression is sometimes called "echo stacking".
An "echo selection" circuit known from German Pat. No. 2,204,352 allows only the first received echo to pass, but inhibits all following signals, including the secondary echo. This circuit comprises a flip-flop which is switched by the first echo signal and remains in the switched position for the rest of the sounding period, thereby inhibiting the receiver circuit from receiving signals following the first echo.
British Pat. No. 1,500,068 shows a blanking circuit wherein echo signals from only a preselected depth range are allowed to pass, and wherein the beginning of said depth range is made dependent on the last measured depth. This blanking circuit forms what is sometimes called a "tracking window".
Finally, British Pat. No. 1,356,019 shows a circuit which establishes a "tracking threshold." The incoming echo signal is compared with a threshold reference signal, and an output signal is generated only if the incoming signal exceeds the threshold. The threshold reference signal is derived from the last received echo for which a corresponding voltage is stored by means of a capacitor. In order to allow the tracking threshold to follow echo signals of decreasing amplitude, the charge on the capacitor is reduced a predetermined amount between succeeding sounding periods.
Although each of these circuits has certain advantages, some problems remain unsolved. The echo stacking circuit distinguishes well between the target echo and interference signals, but may allow secondary echos to pass. Further, a phase error is generated by the delay of the echo stacking operation and causes a depth or distance measuring error. The echo selection circuit suppresses only signals received after receipt of the first significant signal during a sounding period, the first significant signal generally being the primary target echo signal. The tracking window circuit excludes only those signals which lie outside a predetermined portion of the measuring range. The tracking threshold circuit suppresses only those signals which are lower in amplitude than the expected target echo signal.
The present invention combines certain features of the above-mentioned circuits into a receiver circuit leading to error-free selection of the primary target echo without causing a phase error in a recorded representation of the echo.